1. Field of the Invention
This invention relates to a radiation image readout method in which a stimulable phosphor sheet carrying a radiation image stored therein is exposed to a stimulating rays which causes it to emit light in the pattern of the stored image, and the emitted light is photoelectrically read out by a photodetector, and an apparatus for carrying out the method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to such radiation as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible rays, light is emitted from the phosphor in the pattern of the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined patent publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and read-out system. Specifically, the stimulable phosphor formed on a sheet is first exposed to a radiation transmitting through an object to have a radiation image stored therein, and is then scanned with a stimulating ray such as laser beam which causes it to emit light in the pattern of the stored image. The light emitted from the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image on a recording medium such as photographic light-sensitive material or on a display such as cathode rays tube (CRT).
This radiation image system using the stimulable phosphor sheet is advantageous over the conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range of radiation exposure and further in that the electric signal used for reproducing the visible image can be freely processed to improve the image quality for viewing, particularly diagnostic purposes. In more detail, since the amount of light emitted upon stimulation after the radiation energy is stored in the phosphor varies over a very wide range in proportion to the amount of energy stored therein, it is possible to obtain an image having desirable density regardless of the amount of exposure of the phosphor to the radiation by reading out the emitted light with an appropriate read-out gain and converting it to an electric signal to reproduce a visible image on a recording medium or a display. The electric signal may further be processed as desired to obtain a radiation image suitable for viewing and diagnostic purposes. This is very advantageous in practical use.
As mentioned above, in the radiation image system using a stimulable phosphor, deviation of the level of the radiation energy stored in the stimulable phosphor from a desired level can easily be compensated by setting the read-out gain to an appropriate value when photoelectrically reading out the light emitted from the stimulable phosphor upon stimulation thereof. Therefore, the quality of the reproduced radiation image is not adversely affected by a change in radiation does due to fluctuating tube voltage or MAS value of the radiation source, a variation in the sensitivity of the stimulable phosphor or the photodetector, a change in radiation dose according to the condition of the object, or a change in the radiation transmittance according to the object etc. Further, it is possible to obtain a desirable radiation image even when the radiation dose to the object is reduced. Further, it is possible to obtain a radiation image having high image quality of high contrast, high sharpness and low noise etc. by once converting the light emitted from the stimulable phosphor into an electric signal, and processing the electric signal as desired. Particularly, when the radiation image is used for medical diagnosis, it is possible to obtain a radiation image processed in accordance with the portion of a human body such as the heart, the chest etc. and improve the diagnostic efficiency and accuracy.
However, in order to eliminate various influences based on the fluctuation of radiographic exposure conditions and/or obtain a radiation image having a high image quality or a high diagnostic efficiency and accuracy, it is necessary to investigate the image input condition of the radiation image stored on the stimulable phosphor sheet such as high radiation dose recording or the image input pattern, which is determined by the radiographic method such as portion image (e.g. chest and abdomen), plain image or contrasted image radiographing, before reproducing the radiation image to a visible image, and appropriately adjust the read-out gain or appropriately process the electric signal based on the investigated image input condition or the image input pattern. The image input condition and the image input pattern will hereinafter be simply referred to as the image input information when they should be expressed generically. It is also necessary to determine the scale factor to optimize the resolution according to the contrast of the image input pattern.
The investigation of the image input information may be conducted prior to the visible image reproduction by use of the method disclosed in U.S. Pat. No. 4,284,889, which is based on the finding that the amount of light instantaneously emitted from the stimulable phosphor sheet upon exposure thereof to a radiation is proportional to the amount of the radiation energy stored in the stimulable phosphor. In this method, image input information is investigated by detecting the instantaneously emitted light, and an appropriate signal processing is conducted based on the image input information in order to obtain a visible radiation image having an improved image guality, particularly a high diagnostic efficiency and accuracy. With this method, since it is possible to appropriately adjust the read-out gain, select an appropriate scale factor, or conduct an appropriate signal processing, a radiation image suitable for viewing and diagnostic purposes can be obtained regardless of fluctuation of the radiographic exposure conditions. However, since the recording of radiation image to the stimulable phosphor sheet and read out of the recorded image from the stimulable phosphor sheet are usually performed at different location, a signal transfer system must be formed therebetween, necessitating a complicated apparatus and a high cost.
Further, U.S. Pat. No. 4,276,473 discloses a method of estimating the image input condition or image input pattern of a radiation image stored in the stimulable phosphor by positioning a non-stimulable phosphor in the vicinity of the stimulable phosphor sheet, and detecting the light emitted from the non-stimulable phosphor upon exposure thereof to a radiation by use of a photodetector. However, this method also has the same drawback as that of the method disclosed in aforesaid U.S. Pat. No. 4,284,889. Further, since the stimulable phosphor itself for recording the radiation image is not used as the detecting means for the image input information and it is only an indirect detecting method, it is impossible to obtain the image input information which is sufficiently reliable.
Various experiments conducted by the inventors revealed that a radiation image suitable for viewing, particularly diagnostic purposes can be obtained (regardless of fluctuation of the radiographic exposure conditions) by conducting in advance a read-out operation for investigating the image input information of a radiation image stored in a stimulable phosphor (hereinafter referred to as the preliminary read-out) by use of a stimulating rays having stimulation energy lower than stimulation energy of a stimulating rays used in a read-out operation for obtaining a visible image for viewing and diagnostic purposes (hereinafter referred to as the final read-out), thereafter conducting the final read-out. In the final read-out, the read-out gain is adjusted, and/or the scale factor is determined, and/or the image processing conditions are determined appropriately based on the image input information obtained by the preliminary read-out. The read-out gain and the scale factor are together referred to as the read-out conditions. Hitherto it was considered to be necessary to detect as much light as possible with a photodetector, as described in U.S. Pat. Nos. 4,258,264 and 4,302,671, and U.S. Pat. No. 4,346,295 (DE-OS No. 2,951,501), since the amount of light emitted from the stimulable phosphor upon stimulation thereof by a stimulating rays is very small even when the stimulable phosphor having the highest sensitivity among those available was selected. In view of these state of arts, since it is beyond the imagination of the skilled in the art to dissipate intentionally the radiation energy stored in the stimulable phosphor for the purpose of only investigating the image input information, the above findings is unexpected.
On the basis of these findings, the inventors proposed in Japanese patent application Nos. 56(1981)-165111, 56(1981)-165112, 56(1981)-165113, 56(1981)-165114 and 56(1981)-165115 a method of and apparatus for reading out a radiation image in which, before conducting the final read-out for obtaining a visible image for viewing and diagnostic purposes, the preliminary read-out is carried out to investigate the image input information of the radiation image stored on the stimulable phosphor sheet by use of a stimulating rays having stimulation energy lower than stimulation energy of a stimulating rays used in the final read-out.
The stimulation energy referred to in this invention means the effective energy of the stimulating rays which the stimulable phosphor sheet receives per unit area.
In the method just described above, the stimulation energy of the stimulating rays applied to the stimulable phosphor in the preliminary read-out should be lower than the stimulation energy of the stimulating rays used in the final read-out. As the ratio of the stimulation energy of the stimulating rays in the preliminary read-out to the stimulation energy of the stimulating rays in the final read-out increases near to 1, the amount of the radiation energy remaining in the stimulable phosphor after the preliminary read-out decreases. It has been found that, when the ratio is smaller than 1, it is possible to obtain a radiation image suitable for viewing and diagnostic purposes by appropriately adjusting the read-out gain. However, in order to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy, the aforesaid ratio should preferably be as small as possible insofar as the image input information of the radiation image stored in the stimulable phosphor can be detected sufficiently to determine the read-out conditions or the image processing conditions, that is, insofar as the light emitted from the stimulable phosphor in the preliminary read-out can be detected sufficiently for the above-mentioned purposes. Thus, the aforesaid stimulation energy ratio should generally be 50% or less, preferably 10% or less, more preferably 3% or less. The lower limit of this ratio is determined according to the accuracy of the system for detecting the light emitted from the stimulable phosphor in the preliminary read-out.
In order to make the stimulation energy of the stimulating rays in the preliminary read-out smaller than the stimulation energy of the stimulating rays in the final read-out, it is possible to use any known method. For example, the output level of the laser source used in the preliminary read-out may be decreased, the beam diameter of the laser beam may be increased, the scanning speed of the laser beam may be increased, or the moving speed of the stimulable phosphor sheet may be increased.
In the above described method, since the image input condition of a radiation image stored in the stimulable phosphor can be investigated in advance, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy regardless of fluctuation of the radiographic exposure conditions by adjusting the read-out gain based on the investigated image input information without using a read-out system having a wide dynamic range. Further, since the image input pattern of the radiation image stored in the stimulable phosphor can be investigated in advance, it is possible to obtain a radiation image having an improved image quality, particularly a high diagnostic efficiency and accuracy by processing the read-out electric signal suitable according to the image input pattern, and/or by optimizing the scale factor. It is also possible to reduce the read-out time by omitting the final read-out for a portion of the stimulable phosphor carrying no image.
In the above-mentioned method, since the radiation energy stored in the stimulable phosphor fades with time, the interval between the preliminary read-out and the final read-out is considered to have to minimized in order to effectively use the image input information obtained in the preliminary read-out for the final read-out. When the difference between the amount of the radiation energy stored at the time of preliminary read-out and that at the time of final read-out is 10% or less, it is possible to obtain a radiation image having a practically sufficient image quality, particularly sufficient diagnostic efficiency and accuracy by detecting the image input information of the radiation image stored in the stimulable phosphor and setting the final read-out conditions and/or the signal processing conditions by use of the detected image input information. Accordingly, it is considered preferable that the preliminary read-out be conducted within one hour before the final read-out is started.
However, it was found that the fading speed of the intensity of light emitted from the stimulable phosphor when the stimulable phosphor is exposed to a stimulating rays having a predetermined intensity generally differs according to the time elapsed after a radiation image is recorded in the stimulable phosphor. Therefore, if the time interval between the radiation image recording and the preliminary read-out varies, it is not always possible to correctly estimate the amount of light emitted from the stimulable phosphor upon stimulation thereof at the time of the final read-out based on the image input information obtained by the preliminary read-out and to select optimal final read-out conditions only by maintaining the time interval between the preliminary read-out and the final read-out constant. FIG. 1 shows the decay characteristics of the amount of light emitted from the stimulable phosphor upon exposure thereof to a stimulating rays having a predetermined intensity with respect to the stimulation time duration. In FIG. 1, the curve A shows the decay characteristics obtained when the exposure of the stimulable phosphor to the stimulating rays is started 30 seconds after a radiation image is recorded therein, the curve B shows the decay characteristics obtained when the exposure thereof is started one hour after the radiation image recording, and the curve C shows the decay characteristics obtained when the exposure is started four hours after the radiation image recording. The vertical axis of the graph indicates an amount of light emission normalized to 100 at the initial values. As shown in FIG. 1, the fading curve obtained by exposing the stimulable phosphor to a stimulating rays having a predetermined intensity changes according to the interval between the radiation image recording and the exposure to the stimulating rays. Accordingly, only by maintaining the interval between the preliminary read-out and the final read-out constant, it is not always possible to correctly estimate the amount of light emitted from the stimulable phosphor upon stimulation thereof at the time of the final read-out based on the integrated value of the light emission amount determined by the preliminary read-out.